Chris,

I wish I had really hard numbers on the benefit.  If you read some of the stuff from suppliers, you could get the idea that there is an "up to" 20 percent improvement at altitude.  I don't think that the average user will see that much, but the benefit is still substantial.  The engine will produce most power if the spark is adjusted so that the peak cylinder pressure occurs at about 16 degrees after top center (ATC) and that is pretty much independent of engine type and operating condition.  The problem is that the flame speed varies, depending on a number of parameters.  Probably the only two that are pertinent are manifold pressure and mixture.  You might think that engine speed is important (higher speed gives the flame less time to propagate), but the turbulence in the chamber increases as the engine speed increases and that speeds up the flame travel enough or nearly enough to negate that effect.

 

The flame travels essentially by "jumping" from one "treetop" of fuel to the next, so when the fuel molecules are further apart the flame travel is slower.  That delays the point at which maximum cylinder pressure occurs.  So naturally, lower manifold pressure results in slower flame speed and leads to the requirement for more spark advance.  The same is true for leaner mixtures as that increases the distance between fuel molecules.  Somewhat counter-intuitive is that the same thing happens with richer-than-stoichiometric mixtures, but that's a different topic.  The peak flame speed occurs at roughly stoichimetric mixtures, which turns out to be about 50 degrees rich of peak.  And the important thing is that the flame travel will slow progressively faster and faster (does that make sense?  I hope so) as the mixture gets lean or the manifold pressure is reduced.  The reason is that after TDC the volume in the combustion chamber is increasing and that by itself slows the combustion (more distance between molecules).  So if the combustion doesn't come close to completion soon enough it will take a long, long time.

 

The common electronic ignition systems advance the spark as a function of only manifold pressure - at least as far as I know.  At altitudes above 12,000 feet you can expect that the timing will be several degrees advanced from that at sea level.  That gives a certain benefit.  I think the real improvement comes from running LOP at high altitude.  If you try to run LOP at very high altitude without the extra spark advance the power output will drop faster than one might expect, essentially falling off the cliff.  So yes, if you want to run at, say, 15,000 AND LOP the extra spark advance could improve the efficiency of the engine by maybe 20 percent.  In this case I say "efficiency" as opposed to power because the LOP operation will reduce the power output - the spark advance will just keep it from reducing as much.

 

In theory, at least you could then run at a fixed IAS at a higher altitude than you could otherwise and reduce the fuel consumption by that 20 percent.  But if you want to go as fast as possible at a given altitude you won't be running LOP and then I suspect the improvement in speed at that fixed altitude could be about 2 percent.

 

It's not a simple subject, but I hope I've shed a little light on it.  How much improvement can you expect by running only one electronic ignition?  I've been told roughly half.

 

Gary Casey - sorry about the long post.

 

From chris:

thanks,